Bearing and gear failures within gas turbine engines, helicopter gear boxes, and many other types of machinery are typically preceded by the production of wear-related debris. Such debris is usually produced well in advance of a catastrophic failure of a defective component. The wear debris is typically carried by the machine's lubricating oil toward a sump or a filter within the machine. The debris can be intercepted and collected by a chip detector as it is being transported in this manner. Periodic inspection of the chip detector can thus provide an indication of an impending component failure.
Two types of chip detectors are commonly used: magnetic plugs (also referred to as "chip collectors") and electric chip detectors. Chip collectors include a magnetic probe mounted inside a self-closing valve. The magnetic probe is periodically removed from the host machine, i.e., the machine in which the chip detector is utilized, and visually inspected for any accumulation of debris. The self-closing valve inhibits the leakage of lubricating oil from the host machine when the probe is removed for inspection. Inspection intervals usually range from 50 to 200 operating hours. This interval may be shortened when wear debris begins to appear on the probe.
Electric chip detectors also collect wear debris. In addition, electric chip detectors provide an external, electrically-generated indication of the presence of such debris. Electric chip detectors do not require inspection at pre-determined intervals. These types of chip detectors are usually removed and inspected whenever an external signal (a so-called "chip light") has been activated. Electric chip detectors, like chip collectors, usually have a self-closing valve that permits withdrawal of the magnetic probe with little or no loss of lubricating oil.
Many types of machines utilize separate bearing compartments, gear boxes, and gear modules. Such machines often incorporate multiple chip detectors to identify the location of an incipient component failure. This methodology allows a defective component or module to be replaced (as opposed to replacing the entire machine). The regular inspection of multiple chip detectors can be a time consuming maintenance requirement if the detectors are not specifically designed for quick removal and reinstallation.
Chip-detector probes are commonly mounted using a quick-disconnect, bayonet-type locking mechanism. Bayonet-type locks typically comprise two or three locking pins disposed on a surface of the probe. The pins engage an equal number of grooves formed in a housing fixed to the host machine. The grooves terminate in detents. The probe installation process involves pushing the probe into the housing against a spring force, and then twisting the probe so that the pins engage the helical grooves. The spring subsequently locks the probe in place by urging the pins into the detents.
Bayonet-type locking mechanisms have a number of serious shortcomings that have subjected aircraft to costly and potentially dangerous service disruptions, e.g., in-flight engine shutdowns. One such shortcoming involves excessive wear of the pins and grooves that retain the probe. This problem stems from the limited contact area of the pins. More particularly, the relatively small pins concentrate the probe retention forces over a very limited area, i.e., about one-half of the circumference of the pins. This force concentration, combined with the vibration normally generated by most machinery, causes substantial wear in the pins and the grooves. Such wear has caused pin and groove failures in extreme cases, leading to ejection of the chip detector and an ensuing loss of lubricating oil.
Bayonet-type locks also present manufacturing-related drawbacks. Specifically, the retaining pins are typically pressed into a portion of the probe. The pins are often staked, i.e., mechanically deformed, after the pressing operation to further secure the pins in place. The pressing operation usually induces high stresses around the pin holes. These stresses, if excessive, can cause the material around the pin holes to yield, resulting in a loss of the press fit and a potential liberation of the pin. Furthermore, the staking operation cannot be controlled with a high degree of precision, and the results of the staking operation are not easily inspected. Hence, the use of press-fit retaining pins presents quality-control issues. Furthermore, press-fit pins increase the overall parts count of the chip detector.
Bayonet-type locks also present operational disadvantages. Specifically, a bayonet lock does not provide a direct visual indication that the locking means have fully and properly engaged. This disadvantage stems from the fact that the retaining pins and grooves are not visible to the installer of the probe, i.e., the bayonet plug is a so-called "blind assembly." This characteristic increases the possibility of an in-flight loss of the probe due to improper installation. Furthermore, the grooves of a typical bayonet-type lock cannot be easily inspected when the housing is installed in the host machine. In particular, the contact between the pins and the grooves is usually borne by the underside of the groove. This portion of the groove cannot be directly viewed by maintenance personnel. Hence, a proper inspection of the housing requires removal of the housing, or the use of mirrors or other devices that provide a visual image of the underside of the groove.
Blade-type locks are another type of mechanism commonly used to mount and secure chip-detector probes. Blade-type locks incorporate a pair of thin and substantially flat retaining members, or blades, fixed to the probe. Each blade engages a corresponding slot machined into the probe housing, thereby securing the probe to the housing.
Blade-type locks are subject to the above-noted operational disadvantages associated with bayonet plugs, i.e., no direct visual indication that the locking means have fully and properly engaged, and relatively difficult inspection procedures. Furthermore, the slots of a blade-type lock cannot be formed at an oblique angle in relation to the probe centerline without considerable difficulty or expense. Hence, the blades cannot be angled relative to the probe centerline. This limitation prevents the blade-type lock from being configured to automatically eject the probe from the housing in the event of improper, i.e., incomplete, installation (the relationship between an angled retaining means and probe ejection is explained in detail below).
The above-described problems have been apparent for many years. Thus, a long-felt need exists for a chip-detector locking mechanism that provides a direct visual indication that the locking means have fully and properly engaged. The locking mechanism should be capable of being inspected a minimal amount of effort. In addition, the locking mechanism should eject the probe automatically if the probe is not completely installed in its housing. Furthermore, the locking mechanism should be resistant to vibration-induced wear. The locking mechanism should provide these advantages without adding substantially to the parts count or the manufacturing complexity of the chip detector.